I translated the rules for Treibball from German to English some weeks ago for a trainer friend w/border collies who wants to teach a Treibball class. Also some basic notes on training a dog for it. Here are both texts, in German and in English:

Translation of the paragraph with the rules fromhttp://hundherum.foren-city.de/topic,340,-treibball.html

The playing field has the size of half a soccer ball field and a regular goal (could also be a hobby sized goal). At the start, the eight colored gym balls are arranged in a triangle shape (similar to billiard). The dog's task is to push the balls into the goal as fast as possible. To make it more challenging, the dog has to drive the balls in a given sequence. For example, first the blue, then the red, etc. are to be driven into the goal. The handler may only move within a given radius around the goal and can direct his [sic] dog with whistles and commands from there. Penalty time is due for yelling at the dog or otherwise putting pressure on the dog. Adding narrow passes/bottle necks and water ditches add degrees of difficulty to the task "but that is for the already advanced. What matters at the beginning is that the dog learns how to push the ball to the right, left, forward, and backward. And that isn't all that easy!" emphasizes Evelyn Streiff who organized the first ball herding seminar in Switzerland on... There she assisted Jan Nijboer, inventor of ball herding ..., and future seminars will be led by her.

First step is to get the dog used to the gym ball. Many dogs initially react with insecurity to the gym balls and bark at them. Therefore one should never let a ball roll towards a novice dog. Handler should not allow the dog to start the game on its own.

Initially the dog needs to learn to stay and down-stay on commandy, even right next to the balls. Once the dog has that one can start training how to push the ball to the left, right, fwd, and bwd

The balls that I see used in Treibball in Germany are air filled 'fitness balls' or 'exercise balls' (Gymnastikbaelle), 22" to 34" (55 cm to 85 cm) in ø, depending on the dog's size. Roughly the height of sheep! I searched for them on ebay in the US and found plenty.

But unless you are competing, you can of course adapt the rules, invent new ones, and use a smaller ball if that works better for your dog. As you say, any ball bigger than the dog's mouth should work.

In Germany some people use 'piglet balls' (Ferkelbaelle), smaller than the exercise balls. They were originally developed as enrichment for bored piglets in stables, and are very sturdy.

Btw, the fitness balls used for Treibball are very popular for people use in Germany and Switzerland. You can sometimes even get cheap ones at the supermarket around the corner. The quality ones also make great seats for long hours at the desk. They make for an upright posture without rigidity. Sitting on them one moves subtly all the time to keep balance. Prevents RSI and muscle tensions. No slumping while seated on a fitness ball. Might want to remove the dog saliva first, though!

Our exercise ball was about $10 from Target. It's pretty large, taller than Tara for sure. I saw some smaller balls today, about half the diameter of my exercise ball but still bigger than a dog's mouth, for only $2.88- think the kind you see in grocery stores in the big wire cages.